Stable lead anodes



E. J. PARS! ETAL STABLE LEAD ANODES Original Filed May 22, 1962 April22, 1969 |NVENTORS= EDGARDO J. PARSI, ALBERT SZCZUR,J-R.

ATTORNEY United States Patent U.S. Cl. 20429 8 Claims ABSTRACT OF THEDISCLOSURE A method of forming an anode for an electrolytic cellcomprising adding discrete particles of a noble metal to a lowtemperature lead base melt, cooling to solidify without the particlesmelting, cleaning and then anodizing the lead anode body.

This application is a division of Ser. No. 196,758, filed May 22, 1962,now Patent No. 3,284,333.

This invention relates to anodes as articles of manufacture, the methodof making the same, and their use in electrolytic systems. Moreparticularly, it is concerned with a method for fabricating anodescomprising more than one metal component, but one component beingselected from the platinum group. Specifically, it relates tobielectrodes to be used as non-corroding anodes in the electrolysis ofelectrolytic solutions containing chloride ions. More specifically, itrelates to the use of lead base metal anodes in caustic-chlorine cells.

In electrochemical processes such as electrolysis, electrodialysis andin the field of cathodic protection, it is a matter of primeimportanceespecially where the anode comes in contact with a solutioncontaining chloride ionsto properly select the materials from which tomanufacture such anodes. In these processes there are only a few knownmaterials which may eiiectively constitute the anode because mostmaterials, while acting as an anode, are susceptible to intensecorrosion by oxygen and chlorine gas which are evolved at said anode. Ifonly chemical characteristics of the metal were to be considered in theselection of a suitable anodic material, the metals of the platinumgroup would be the universal choice because of their high resistance tocorrosion. However, the high cost of these precious metals prohibitstheir extended commercial use in most processes. In recent years mucheffort has been directed to the construction of anodes for use inelectrolytic processes for coating the surface of an electrolytic valvemetal (such as tantalum or titanium) with a precious metal likeplatinum. Although the use of such anodes for commercial use has shownpossibility, it is still quite expensive to fabricate them anddifiiculty has been encountered in producing a precious metal coatingwhich will adhere to the base metals with suificient tenacity. From timeto time many other materials have been tried as a substitute for theprecious metals, but the primary anodic material employedtodayespecially in the caustic-chlorine industryis still graphite. Manydisadvantages accompany the use of graphite anodes since the materialundergoes continual disintegration and must be replaced ratherfrequently; this causes interruption in the electrochemical processwhich is a costly operation. Also in the manufacture of chlorine gas byelectrolysis of chlorine salt solutions, the gas becomes contaminatedwith traces of carbon dioxide which results from the anodic oxidation ofthe graphite.

A continuous search has been made for an anode material which has thedesirable anti-corrosive characteristics 3,440,149 Patented Apr. 22,1969 of the platinum metals, but which lacks their prohibitive cost. Ithas long been known that lead can be used as an inexpensive andsatisfactory anode especially when employed in a sulfuric acid medium.During the anodic process the lead surface of the anode is oxidized tolead peroxide (Pb02) which is brownish in color and which possesseschemical stability with high electronic conductivity. Thus, a PbO /Pbsystem makes an ideal anode provided the covering of lead peroxideremains in contactwith the underlying lead and provided it is reformedwhen a discontinuity occurs in the peroxide coating. However, where suchan anode is used in the presence of chloride ions, the lead peroxidefilm cannot be maintained on the entire lead surface; hence, thechloride ions penetrate the peroxide film and form a non-conductinglayer of lead chloride on the underlying lead base. This causes the leadperoxide to become insulated from said lead base. Under these conditionsit has been found that at a constant current density, the voltage dropincreases rapidly, and eventually the anode completely disintegrates.

Heretofore it was known that the anodic corrosion of lead in a chlorideelectrolyte could be prevented by introducing a single platinum wireinto the surface of a one cm. lead anode (Anodic Behavior of a Lead-Platinum Bielectrode in Chloride Electrolytes, E. J. Lit tauer and L. L.Shreir, Platinum Metals Review, 1961). When this bielectrode becomesanodic in a medium containing chloride ions, a film of lead chloride isfirst produced. However, it is rapidly replaced with stable leadperoxide which initially forms at the interface of the lead and platinummicroelectrode. The platinum wire, penetrating the surface of the leadanode, acts as a nucleus for the formation of lead peroxide in thevicinity of said platinum wire. When the peroxide completely covers thecomparatively small anode surface, the only anodic reaction taking placeis the oxidation of chloride ions to chlorine gas with theperoxide-coated base metal remaining stable.

An important advantage of the present invention over the prior art isthe use of small discrete particles of noble metal (in preference to"the relatively large platinum wire of the prior art). The discreteparticles will give better protection since many more microelectrodesare present to act as nuclei for more rapid and complete coverage by theperoxide coating. Additionally, the platinum wire method of protection,as shown in the prior art, may be effective when employed with extremelysmall anodes of 1 square centimeter or less, but proves inefiective whenused in conjunction with larger, con ventional electrodes of, forexample, 1 square foot or more. This disadvantage has been eifectivelyovercome by the present invention.

It is therefore the object of this invention to provide an improved andmore simple method for manufacturing bimetallic anodes of any size whichare stable in electrolytic processesespecially those involvingchloridecontaining solutions.

Another object is to make an improved bimetallic anode which comprisesnumerous microelectrodes dispersed throughout the lead anode, or only onthe surface.

A further object is to achieve these results in a simple, rapid, andeconomical manner.

Further objects and various advantages of this invention will beapparent from a study of this disclosure, the drawings, and appendedclaims.

FIGURE 1 is a preferred embodiment of the invention and represents aperspective view of -a fully-formed bielectrode anode constructed inaccordance with the present invention; and

FIGURE 2 is a side elevational view of the bielectrode 3 in crosssection taken in a plane represented by the line 2-2 of FIGURE 1.

The drawings illustrate an anode in sheet form 1 which is comprised of alead base metal 5 having imbedded therein noble metal particles 6 and 4.The upper portion of the drawings represents the active working area ofthe anode which comprises a solid adherent layer or coating of lea-dperoxide 3 and surface microelectrodes 6 (finely divided noble metalparticles). Below the coated area are sub-surface microelectrodes 4 of anoble metal embedded within the lead base metal 5. It can be appreciateda bielectrode could be constructed which would contain only surfacemicroeleotrodes since it would only be necessary for the requiredprotection to have noble metal particles on the anode surface. Themanner of producing the above-illustrated bielectrode types ishereinafter more fully described.

The term platinum metal" as used herein means the precious metals inGroup VIII of the Periodic Table; that is, the term includes thefollowing metals: ruthenium, rhodium, palladium, iridium, platinum,osmium, and alloys of these metals. Platinum, however, will be referredto hereinafter as representative, and as the preferred metal of GroupIII for the purposes of this invention. It is also intended that theterm lead as used herein include the commercial form containing a widerange of minor impurities. The present invention is also applicable toelectrodes having base metals comprising alloys of lead and antimony aswell as lead and silver.

The process of this invention involves imbedding numerous discreteparticles of a noble metal, or a combination of noble metals, on thesurface and/or throughout the lead anode. However, where the bielectrodecontains only surface microelectrodes, it is possible for these to flakeotf or to become dislodged during operation. This would result in a leadanode having no further protection; thus, eventual disintegration wouldoccur. However, this does not occur in a bielectrode constructed withnoble metal particles throughout since the deeper imbedded platinumparticles would eventually become exposed to the surface and again actas protective microelectrodes, preventing further corrosion of the basemetal.

During the manufacture of the bielectrode, the platinum particles areadded to molten lead-the temperature of which is kept below the meltingpoint of platinum. The mixture is stirred vigorously to obtain an evendistribution of the discrete platinum particles within the molten lead.When only surface microelectrodes are desired, the particles are added,for example by sprinkling, to the surface of the liquid lead. Afteraddition, the lead is allowed to cool until solid. The particles ofplatinum should then be uniformly small in size ranging from a finepowder to a dimension not above 50 mesh. The preferred material formicroelectrodes is powdered, spongy platinum since it possesses largesurface areas and is relatively inexpensive when compared with platinumblack or solid platinum metal. The more finely divided the platinum, themore efiicient for the instant purpose. Because of economicalconsiderations, the bielectrode should not contain more than one percentby weight of the precious metal. Satisfactory anodes have been preparedcontaining as little as 0.01% platinum, but the preferred amount forpurposes of this invention is between 0.01% and 0.1% by Weight.

As can be appreciated, bielectrodes of any desired size or shape can bemade, for example by molding, rolling, hammering, etc. It should also benoted that the present article of manufacture is not limited to anyspecific shape or form so long as portions of the platinum metal areexposed to the electrode surface. It is, therefore, recommended that,prior to any coating operation, the surface of the bielectrode be sandedto obtain a smooth surface and to expose more platinum parti cles on theelectrodes surface. Prior to anodically coating the electrode with leadperoxide, it is preferable 4- that the electrode be pickled in acid.This pickling treatment minimizes any tendency of the lead peroxide toflake off during the anodic coating operation, and thus produces a moreadherent and smoother coating of peroxide.

The electrolytic coating operation is preferably carried out byemploying the bielectrode as an anode in the electrolysis of a chlorideor sulfate solution. The concentration of the chloride ions can rangefrom 0.1 normal to 1.3 normal; however, the preferred concentration isfrom 0.4 to 0.6 normal. In concentrations far below 0.1 or above 1.3normal, it is clifficult to obtain a satisfactory coating of leadperoxide. Where the coating operation is carried out in a sulfatesolution, it is preferred that the concentration of sulfate ions be in arange above 0.05 normal. When a direct current is impressed across theelectrolytic cell, a thin peroxide coating forms on the anodic surface.The more numerous the microelectrodes, the more rapid the formation ofthe coating. The thickness of the coating depends primarily on thecurrent density and coating time. Anode current densities of 9Om'illiamps per square centimeter will produce the most satisfactoryprotective coatings-the gerater the current density, the more rapid theperoxide formation.

The following examples are illustrative of preferred modes of carryingout the invention and are not intended to be limiting.

Example 1 Commercial lead was heated to 340 C. in an iron ladle. Whenmolten 0.1% by weight of spongy platinum particles (which had passedthrough a US. Standard 200 mesh sieve) was added slowly while vigorouslystirring the molten mass. Stirring was accomplished with a WaringBlendor, which dispersed the platinum particles into the bulk of thelead medium. During the stirring operation, the temperature of the masswas kept at 340 C. to prevent solidification. The molten lead,containing the dispersed, solid platinum particles, was then poured intoa 12" x 12 fiat square mold and allowed to cool and solidify. Then thecast bielectrode was sanded smooth and pickled in concentratedhydrochloric acid. The pretreated biclectrode was then made anodic in anelectrolytic bath containing a 0.5 normal aqueous solution of NaCl. Thecathode material was stainless steel 314. A direct potential wasimpressed across the electrodes equal to a current density of milliampsper square centimeter of anode area. Since chlorine gas was evolved atthe anode and hydroxide at the cathode, it was necessary to add HCl tothe electrolytic solution at various intervals to neutralize thehydroxide formed and to keep the concentrating of the chloride ions at-0.5 normal. After one hour of anodizing, the anode had developed a deepadherent coating of lead peroxide. The bielectrode, prepared and coatedas above, was employed as an anode in an electrodialysis unitdemineralizing 3,600 ppm. brackish water down to 300 ppm. Although thebrackish water contained 2400 ppm. of dissolved NaCl, the anode revealedno appreciable corrosion or increase in voltage drop after four days ofcontinuous operation.

Example 2 An electrode, employing antimonial lead as the base metal, wascast as in Example 1, but was not pickled in acid prior to beinganodized in a 0.5 normal NaCl solution. During the electrolysis theperoxide coating formed, but a small degree of flaking occurred. Thesurface of this same anode was then sanded smooth (to remove anyremaining oxide coating and to expose the platinum particles), pickledin concentrated I-ICl for 5 minutes, and employed once again as ananode. This time the film of lead peroxide formed smoothly and rapidlyand no flaking was noticed. Thus, the preliminary surface treatment ofsanding and pickling assists in producing a solid adherent coating oflead peroxide.

Example 3 A lead-palladium black bielectrode was constructed employing0.01% by weight of the precious metal. The palladium was not dispersedthrough the molten lead but merely sprinkled on, and then scratchedinto, the lead surface. Upon cooling the article was lightly sanded (tosurface expose more of the palladium particles), pickled in sulfuricacid, and anodically oxidized in a 1 normal solution of sodium sulfateat a current density of 90 milliamps per square centimeter of anodearea. When a sufficient coating of peroxide had developed, the finishedarticle was used as an anode in a caustic-chlorine cell operating onsaturated brine solution. A current density of 100 amps per square footof anode area was employed, and after 70 hours of operation, only slightanodic attack was visible.

What we claim is:

1. A method of manufacturing an anode comprising the sequential steps ofadding discrete particles of a noble metal to molten lead base metal,said metal made molten at a temperature below the melting point of thenoble metal, cooling the mass to a solid, submerging the same in asolution of an electrolyte, applying a direct current to said article toanodically oxidize and coat the lead surface with a stable film of leadperoxide.

2. The method of claim 1 wherein the noble metal particles are dispersedthroughout said molten lead.

3. The method of claim 1 wherein the noble metal particles are dispersedonly on the surface of said molten lead.

4. The method of claim 1 wherein said article is anodically oxidized inan electrolyte solution comprising 0.1 to 1.3 normal in chloride ionsusing an anode current density of between 5 and 200 milliamps per squarecentimeter.

5. The method of claim 1 wherein said article is anodically oxidized inan electrolyte solution comprising at least 0.05 normal in sulfate ionsusing an anode current density of 5-200 milliamps per square centimeter.

6. A method of manufacturing an electrode comprising the sequentialsteps of adding discrete particles of a noble metal to molten lead basemetal, said metal maintained molten at a temperature below the meltingpoint of the noble metal, cooling the mass to a solid state, polishingthe surface of the resulting solid particle, pickling said article inacid, applying a direct current to said article to anodically oxidizesaid lead base anode surface in the presence of an electrolyte, saidoxidized coating comprising an adherent coating of lead peroxide.

7. The method of claim 6 wherein the pickling acid employed ishydrochloric acid.

8. The method of claim 6 wherein the pickling acid employed is sulfuricacid.

References Cited UNITED STATES PATENTS 2,305,539 12/1942 Lowry 204-2902,546,548 3/1951 Koster 204-56 2,945,791 7/1960 Gibson 204290 XR HOWARDS. WILLIAMS, Primary Examiner.

WILLIAM B. VAN SISE, Assistant Examiner.

US. Cl. X.R.

